The instant invention pertains to the art of producing tunable far-infrared laser radiation.
Although discrete frequency laser lines are now abundantly available throughout the far-infrared spectral region, as is discussed by T. Chang, "Optical Pumping in Gases", pp. 215-272, in Topics in Applied Physics, Vol. 16, Nonlinear Infrared Generation, Editor: Y. R. Shen, practical tunable coherent sources are still lacking.
Attempts to produce tunable far-infrared laser radiation using the stimulated-electronic Raman effect, as evidenced in Optics Communications, February 1976, pp. 256-258 and Journal of Physics B: Atomic and Molecular Physics, 1976, pp. L77-L81, have problems because this type of Raman effect is a resonance phenomenon which requires a tunable pump laser. This causes the usable output radiation to be weaker as the pump laser is tuned further away from the pumping transition. A further problem with the stimulated-electronic Raman effect is that the emission at the "tuned frequency" is usually accompanied by an undesired emission at the single photon transition. This second emission has essentially the same small signal gain as the tunable output.
Attempts to tune molecular far-infrared lasers using DC Stark tuning, as evidenced in Infrared Physics, Vol. 16, p. 453, 1976, IEEE Journal of Quantum Electronics, Q.E. 13, p. 481, 1977 and The Applied Optics, Vol. 16, p. 1893, 1977, suffer from the fact that (a) all levels of the system are affected by the electric field, including the pump transitions, (b) the systems require far-infrared resonators which would have to be tuned in step with the Stark field, (c) the maximum possible Stark shift as limited by gas breakdown is relatively small, and (d) is almost all molecular far-infrared lasers, the laser transition splits into many Stark components.
The prior art contains references to fundamental investigations of the phenomenon known as the dynamic Stark effect. In studying this phenomenon, investigators have used absorption methods to detect the line shift and to verify experimentally that it coincides with theory. These studies have utilized three-level systems where intense radiation was applied between the bottom and middle levels of the system in order to shift the middle level. The shift of the middle level was probed by measuring the absorption of incident radiation whose energy corresponded to transitions between the middle and upper levels of the system. Although these experimental and theoretical investigations form prior art for the dynamic Stark effect they do not teach its use for producing the unique results obtained by the instant invention.